1. Field of the Invention
The invention is related to the field of optical communications, and in particular, to an optical communication system that implements ring-based restoration by using shared protect links.
2. Description of the Prior Art
FIG. 1 illustrates Synchronous Optical Network (SONET) system 100 in an example of the prior art. SONET system 100 includes optical nodes 101-112 and optical spans 121-144 that are coupled together to form optical rings 151-156 as indicated in the following table.
COUPLEDSPANNODESRING121101 &102151122102 &106151123105 &106151124101 &105151125102 &103152126103 &107152127106 &107152128102 &106152129103 &104153130104 &108153131107 &108153132103 &107153133105 &106154134106 &110154135109 &110154136105 &109154137106 &107155138107 &111155139110 &111155140106 &110155141107 &108156142108 &112156143111 &112156144107 &111156
Optical rings 151-156 each represent a Four-Fiber, Bi-directional, Line-Switched Ring (4-fiber BLSR), although other ring configurations, such as a 2-fiber BLSR, could also be used. Optical spans 121-144 each represent four optical fibers—two working fibers that transfer optical communications around the ring in opposite directions, and two protect fibers that provide protection capacity for the working fibers. Note that spans 127 and 137 occupy physical route 120 between optical nodes 106 and 107. Thus, route 120 includes eight optical fibers for two 4-fiber BSLRs.
FIG. 2 illustrates optical nodes 106-107 in an example of the prior art. Optical node 106 includes Add/Drop Multiplexers (ADMs) 201-204, digital cross-connect 209, and optical links 211-214. Optical node 107 includes ADMs 205-208, digital cross-connect 210, and optical links 215-218. Digital cross-connect 209 is coupled to respective ADMs 201-204 over respective optical links 211-214. Digital cross-connect 210 is coupled to respective ADMs 205-208 over respective optical links 215-218. Digital cross-connects 209-210 and optical links 211-218 provide communication capability between the rings. In some cases, the ADMs and cross-connects at a given node are integrated into one system.
ADMs 201-208 are coupled to optical spans 122-123, 126-128, 131-134, 137-138, 140-141, and 144 to form optical rings 151-156 as indicated in the following table.
ADMSPANRING201122151201123151202128152202127152203133154203134154204137155204140155205126152205127152206131153206132153207137155207138155208141156208144156
Physical route 120 includes optical span 127 of optical ring 152 and optical span 137 of optical ring 155. Thus, optical rings 152 and 155 share physical route 120. Optical span 127 includes work lines 219-220 and protect lines 221-222. Optical span 137 includes work lines 223-224 and protect lines 225-226. In this example, each line represents a fiber in a 4-fiber BLSR, although other line/fiber configurations could be used in other examples.
FIG. 3 illustrates physical route 120 in an example of the prior art. Physical route 120 includes optical span 127 having work lines 219-220 and protect lines 221-222. Physical route 120 includes optical span 137 having work lines 223-224 and protect lines 225-226. Work line 219 has a set of STS-1 signals numbered from #1 to #N, where the term STS-1 #1 represents the first STS-1 signal in work line 219 and STS-1 #N represents the Nth STS-1 signal in work line 219. Lines 220-226 have similar STS-1 configurations. The STS-1 #1's in protect lines 221-222 transport restoration overhead in an active location to initiate restoration in the event of a fault on optical ring 152. Likewise, the STS-1 #1's in protect lines 225-226 transport restoration overhead in an active location to initiate restoration in the event of a fault on optical ring 155.
FIG. 4 illustrates protect line 226 in an example of the prior art. Protect line 226 includes STS-1's #1 to #N. STS-1 #1 includes an overhead portion and a payload portion. The overhead portion includes Section Overhead (SOH) and Line Overhead (LOH). The payload portion includes Path Overhead (POH) and user data. The LOH in STS-1 #1 of protect line 226 is the active location for restoration overhead that indicates when protect line 226 is used to restore optical ring 155. The restoration overhead is typically the K1/K2 bytes in the LOH. Although not shown, the LOH in STS-1 #1 of protect lines 221-222 and 225 are the active locations for restoration overhead that indicates when respective protect lines 221-222 and 225 are used to restore their respective optical rings. Note that the LOH of STS-1 #1 transports the restoration overhead. STS-1 #2 to #N are not used for restoration overhead, but rely on the restoration overhead in STS-1 #1. The LOH also has a Data Communication Channel (DCC) that can be used for messages between line entities.
Those skilled in the art are aware that additional systems, such as Wave Division Multiplex (WDM) equipment, may be included in communication system 100, but these additional systems are omitted for clarity.
Referring to FIGS. 1-4, in operation, optical ring 152 transports optical communications using optical spans 125-128 and optical nodes 102-103 and 106-107. Optical ring 155 transports optical communications using optical spans 137-140 and optical nodes 106-107 and 110-111. If ADM 204 detects a fault on optical span 140, then ADM 204 couples work line 223 to protect line 226 and sets the K1/K2 bytes in the LOH of STS-1 #1 on protect line 226 to indicate that protect line 226 is being used to restore optical ring 155. In response to the K1/K2 bytes in STS-1 #1 on protect line 226, ADM 207 couples protect line 226 to a protect line in optical span 138. In response to the K1/K2 bytes in STS-1 #1 on the protect line in optical span 138, optical node 111 couples the protect line in optical span 138 to a protect line in optical span 139. Thus in response to a fault on span 140, optical node 106 transfers communications to optical node 110 over protect lines in spans 137-139, and the restoration is signaled by the K1/K2 bytes in the LOH of STS-1 #1 on these protect lines.
When optical node 110 detects the fault on optical span 140, then optical node 110 couples the affected work line in optical span 139 to a protect line in optical span 139 (going in the opposite direction). Optical node 110 also sets the K1/K2 bytes in the LOH of STS-1 #1 of the protect line to indicate that the protect line in span 139 is being used to restore optical ring 155. In response to the K1/K2 bytes in STS-1 #1 on the protect line in optical span 139, optical node 111 couples the protect line in span 139 to a protect line in span 138 and sets the K1/K2 bytes accordingly. In response to the K1/K2 bytes in STS-1 #1 on the protect line in optical span 138, ADM 207 couples the protect line in optical span 138 to protect line 225 in optical span 137. Thus in response to the fault on span 140, optical node 110 transfers communications to optical node 106 over protect lines in spans 137-139. The K1/K2 bytes in the STS-1 #1 on these protect lines are set to indicate that the protect lines are being used to restore ring 155 from a fault on span 140.
Ring 152 could implement restoration in a similar fashion. For a fault on span 128, protect lines are used to transfer the optical communications from node 106 through nodes 107 and 103 to node 102, and protect lines are used to transfer the optical communications from node 102 through nodes 103 and 107 to node 106 to restore the faulty span 128. The K1/K2 bytes in the STS-1 #1 on these protect lines are set to indicate that the protect lines are being used to restore ring 152 from a fault on span 128.
There are two primary forms of fault restoration in optical networks—ring-based restoration and mesh-based restoration. Ring-based restoration is typically much faster than mesh-based restoration. The faster ring-based restoration provides a better quality-of-service to the communications user. Ring-based restoration is also typically much less complex than mesh-based restoration. Thus, ring-based restoration can be easier for network personnel to understand and manage.
To provide the faster and simpler ring-based restoration, a protect line is needed for each work line in the prior art. Thus, ring-based protection in the prior art does not allow the work lines in the same physical route to share a single protect line. Mesh-based restoration allows protect capacity to be shared, but mesh-based restoration is much slower and far more complex than ring-based restoration. Unfortunately, there is no effective technique to use ring-based restoration while allowing the rings to share protect lines on shared physical routes.